One of the biggest of course is the generally renewable nature of this source of energy. With the proper location and engineering, maintenance requirements for the source are relatively low and you don't have all the massive cleanup problems you do for coal or nuclear power production.

Beyond renewability, there is also the issue of climate. Today, we worry as much about carbon dioxide output as we do about the things we used to worry about (radioactive waste, impact of coal mining, etc). Hydro power does not release any greenhouse gases and is therefore less likely to contribute to global climate change.

From another angle, consider that the production of cheap and abundant hydroelectric power also attracts industries and businesses that rely on such cheap electricity to thrive. So while you gain irrigation water, erosion and flood control, and power for domestic use that is clean and renewable, you also tend to create jobs. In Washington State, where the Columbia River has been harnessed numerous times, we had thousands of aluminum smelter jobs for decades, and now a Google server farm is located along the river as well.

Hydroelectric energy is cheaper to produce. It does not pollute the environment, and it is readily available. It seems almost wasteful to use other sources, fossil fuels especially, when we have this option available to us instead of the other more wasteful ones.